A closed/unpowered iron door is unaffected as far as i can tell, but if the door is powered and open a villager can use it as if it was a wooden door and open and close it. I also only tested this on double iron doors but both doors are still affected. both doors are connected to the same lever. in the attached images a baby villager walk in one door close it and out the other and close it, without the switched being moved.